I'm afraid it's not currently possible to add IntenseDebate to a WordPress.com blog, my apologies for the inconvenience.

With regards to a splash page, I can suggest that you use a static page as your home page. You can create pages from Pages > Add New in your dashboard, and then decide to set one as your home page form Settings > Reading in your dashboard.

You'll need to be sure to choose another page on which to display your blog posts so that your readers can still see them.